The present invention relates to a method for the manufacture of a thermometer particularly a thermoareometer especially made of glass.
In the same way as therometer, the thermoareometers have been known for a considerable time and form a subvariety of a graduated hydrometer provided with a thermometer. Thermoareometers are known, in which the thermometer bulb serves as a ballast chamber. In addition, thermoareometers are known, which have so-called double balls and below the bulb there is a second part acting as a weight. Nowadays, the thermoareometers which are produced have the ballast filled in and surrounding the thermometric capillary above the bulb.
The manufacture of a good thermometer of precision thermometer, generally of glass, as well as a thermoareometer fundamentally, substantially takes place in the following manner. Firstly, the envelope tube, or the areometer body (base) in an areometer, is preshaped from a hollow glass part or a tubular glass part and in the vicinity of the lower end a constriction is formed, e.g. by means of a rolling machine or the like. A substantially tubular or hemispherical section is generally connected to this constriction. In addition, a capillary is bent in S-shaped manner and its end is generally expanded in funnel-shaped manner, optionally following the formation of a contraction enlargement, and/or a glass ring is fused onto the end. The diameter of the funnel on the capillary or glass ring substantially corresponds to the diameter of the envelope tube constriction. An expansion enlargement is then normally blown into the end opposite to the funnel. The glass blower then takes up the expanded capillary and/or the capillary provided with the glass ring and feeds it into the envelope tube up to the constriction. Accompanied by heating, the glass blower then fuses the funnel or ring to the aforementioned tubular or spherical part of the thermometer or thermoareometer. The fused in funnel or glass ring forms the upper part of the bulb. The capillary projects into the side of the hollow glass opposite to the upper part of the bulb or tube. This is followed by the turning of the envelope tube and a quantity of thermometric liquid is introduced into the tube which corresponds to the desired elevation or pitch of the capillary used. After marking, the uppermost thermometric liquid level in the tube, the liquid is removed and the end of the tube is precisely sealed at the mark giving the lower part f the now closed bulb. The bulb, together with the capillary, are then generally pumped free from air, filled with thermometric liquid, followed by the preadjusting of the thermometer.
The excess thermometric liquid is removed from the capillary by burning off, i.e. heating the liquid in the upper part of the capillary for evaporating the excess liquid. Each individual thermometer or areometer body with capillary must be manually introduced into a burning off apparatus. As a function of the liquid used, more or less toxic vapous are formed (e.g. very dangerous mercury vapours). To ensure that an excessive amount of thermometric liquid is not burnt off from the capillary, each burning off operation must be carefully observed and the particular thermometer or areometer body must be removed from the burning off apparatus in good time. This complicated preadjustment is brought about in that the previosuly formed expansion enlargements vary more in the inner volume and due to the narrowness of the body, cannot be subsequently formed. The capillary is sealed after marking the preadjustment filling level. The thermometric scale carrier, together with the scale, are then introduced into the envelope tube and connected to the capillary in various different ways. The scale is generally supported on the S-shaped bend in the capillary. The thermometer is then sealed at the top, the scale generally being additionally fixed at the seal. The scale is graduated in accordance with the elevation of the capillary. In the case of a thermoareometer, the upper end of the areometer body is constricted and the stem is fused as cylindrically and vertically as possible. The vertical, central fusion is important, so that subsequently the areometer floats completely vertically to the surface of the liquid to be measured and consequently provides the optimum state for a correct reading of the apparatus (measurement).
Furthermore, particularly in the case of a thermoareometer, the weight is introduced into the areometer body, the areometric scale carrier is introduced into the stem and fixed, the adjustments of the areometric scale are carried out, whilst supplementing the balance during adjustment and sealing by means of sealing wax or the like in the vicinity of the weight. Finally, the top of the stem is generally once again manually sealed. With regards to further details on the manufacture of thermoareometers, reference should be made to the aforementioned publication, which describes the procedure in detail and nothing has fundamentally changed with respect thereto.
The decisive point is that a large part and specifically the most difficult part of the manufacture of precision thermometers and areometers takes place manually. Thus, in particular, the connection of the envelope tube and the thermometer capillary can only be carried out by a highly qualified special glass blower, who has had many years experience and who is therefore very well paid. It takes him several minutes to construct the glass tube, expand the capillary, complete the bulb and form the stem top. Even in the case of a good glass blower, it is still impossible to completely avoid a certain amount of waste. Automatic capillary blowing machines cannot be used in the manufacture of such thermometers.